1. Field of the Invention
This invention relates to a laser scanner for use in laser printers and like apparatus.
2. Description of the Related Art
In conventional laser scanners for use in laser printers, an f.theta. lens as a scanning lens and a rotating polygonal mirror as an optical deflector are combined to deflect a laser beam for scanning over the surface of a photoreceptor drum.
The principal object of such laser scanners is to focus a laser beam on the surface of a photoreceptor drum to form a beam spot of a predetermined size in accordance with a required resolution as it scans over the drum's surface at a regular speed.
A prior art example of the f.theta. lens assembly for achieving high resolution is described in Unexamined Published Japanese Patent Application (kokai) Sho 63-253916. This lens assembly consists of two lens elements, one being spherical and the other being aspheric. The aspheric lens is provided with an aspheric surface of rotation asymmetry that has the curvature in a direction normal to the scan direction varied asymmetrically with respect to the central optical axis of the lens. This design is effective in reducing the curvature of field that occurs in a direction normal to the scan direction owing to the fact that the point on which a laser beam is reflected by the rotating polygonal mirror varies asymmetrically on the right and left sides with respect to the center of scanning.
Another prior art example of the f.theta. lens assembly for achieving high resolution is described in Unexamined Published Japanese Patent Application (kokai) Hei 4-60608. This lens assembly consists of two simple lens elements, one of which is provided with an aspherical surface of rotation asymmetry that has its cross-sectional geometry in the main scanning direction varied asymmetrically with respect to the central optical axis of the lens. This design is also effective in reducing the curvature of field that occurs in a direction normal to the scan direction owing to the fact that the point on which a laser beam is reflected by the rotating polygonal mirror varies asymmetrically on the right and left sides with respect to the center of scanning.
These two examples of the prior art make it clear that the higher the resolution that need be achieved by the f.theta. lens, the more significant is the effect that is exerted on the curvature of field in a direction normal to the scanning direction by the asymmetric variation of the point on which a laser beam is reflected by the rotating polygonal mirror; therefore, it is effective to use a lens surface that is asymmetric with respect to the central optical axis of the lens.
As described above in connection with the prior art, the curvature of field that occurs in a direction normal to the scanning direction can be reduced by using a lens surface that is asymmetric with respect to the central optical axis of the lens.
However, this approach of using a lens element of an asymmetric shape presents its own problem if it is combined with another lens element because any error in the relative positions of the two lens elements can cause significant and by no means negligible effects on the curvature of field that would occur in a direction normal to the scanning direction.